Drive circuit for electric motors

ABSTRACT

A method of operating a drive circuit for parallel electric motors is provided. The method includes receiving measurements of stator phase currents in the parallel electric motors. The method includes selecting a target PM motor, from among the parallel electric motors, that generates a largest torque output. The method includes executing a vector control algorithm to generate a complex command voltage vector for the target PM motor. The method includes generating and transmitting a pulse width modulation (PWM) signal based on the complex command voltage vector for controlling an inverter. The method includes operating the inverter according to the PWM signal to supply three-phase alternating current (AC) power to the parallel electric motors.

BACKGROUND

The field of the disclosure relates generally to a drive circuit forelectric motors and, more specifically, a drive circuit that operatestwo or more electric motors in parallel with a single inverter or singledrive device. The electric motors may be permanent magnet (PM) motors orinduction motors.

PM electric motors are operated synchronously in that the rotor turns ata speed that matches the speed at which a rotating magnetic fieldgenerated by the stator turns. The stator and rotor of a PM motor, orsynchronous motor, are energized independently, generally with analternating current (AC) supplied to the stator windings. In contrast,induction motors operate asynchronously, i.e., the rotor turns at aspeed that lags the speed of the stator rotating magnetic field, i.e.,the synchronous speed. The relative speed of the rotor and the rotatingmagnetic field induces the rotor current.

Generally, PM motors are more efficient, but tend to be more complex andoften costlier than their counterpart induction motors. At least some PMmotors are driven utilizing a vector control scheme that independentlymonitors and controls motor torque and motor flux, i.e., monitors rotorposition and phase currents, and independently controls torque currentand flux current via a complex voltage (i.e., a voltage amplitude andphase represented in a complex plane). For a given PM motor, torquecurrent and flux current are controlled over time by a pulse widthmodulated (PWM) signal that controls switching in an inverter thatsupplies, for example, three-phase current to the stator windings. Suchcontrol may be accomplished, for example, using vector control. Athree-phase PWM voltage signal for energizing the stator windings isgenerated based on a complex voltage vector in a rotating rotorreference frame. The complex voltage vector is derived using, forexample, a vector control algorithm executing on a digital signalprocessor (DSP) or other suitable processor for controlling theinverter.

Vector control algorithms are generally known. An exemplary vectorcontrol algorithm begins with measured stator phase currents that aretransformed to the rotating rotor reference frame. The rotating rotorreference frame is derived from the rotor position, which is eithermeasured directly or integrated from a measured rotor speed or inferredthrough mathematical models. For each phase, a rotor flux linkage vectoris estimated based on the stator current vector and the magnetizinginductance of the stator coil. The rotor flux linkage vector gives arotor angle that enables the stator current vector to be converted to a(d,q) coordinate system in the rotating rotor reference frame. The (d,q)coordinate system, sometimes referred to as the flux-torque coordinatesystem, represents a complex current vector with orthogonal componentsalong a direct axis (d) and a quadrature axis (q) such that a field fluxlinkage component of the complex current vector aligns with the d-axisand a torque, or armature flux, component aligns with the q-axis. Oncethe stator current vector is represented in the (d,q) coordinate system,its components may be controlled using traditional scalar control,including, for example, proportional and integral (PI) control, thatproduce a complex commanded voltage vector in the (d,q) coordinatesystem. The complex commanded voltage vector is then converted back tothe original rotating rotor reference frame and is the basis forgenerating a PWM voltage signal for controlling an inverter thatenergizes the stator windings.

At least some motor applications can utilize multiple, smaller and moreefficient motors in parallel to improve output or efficiency. Suchapplications may include heating, ventilation, and air conditioning(HVAC), refrigeration, compression, pumps, or other fluid-movingequipment, as well as electric drives for wheels, gears, belts, or othermechanical loads. Induction motors are often utilized in suchapplications due to their relative simplicity and ability to operateasynchronously, i.e., to allow “slip” between rotor rotation andmagnetic field rotation, thereby simplifying loading of each motor.Conversely, each PM motor in a multi-motor application typicallyrequires a dedicated PM drive to generate the appropriate PWM signal tooperate the motor synchronously for its particular load. Consequently,each PM drive is generally rated for full output power required for theapplication, resulting in higher costs, lower efficiency, and morecomplex configuration and installation. Alternatively, and notably lesspractical, multiple PM motors or induction motors may be combined inparallel with a single drive, but is generally impractical, becauseloading on the various motors is not known or controlled well enough tobalance loads among the parallel motors. Consequently, in suchapplications, the motors operate at varying speeds under varying loadsthat could lead to stability challenges and motor damage, if theyoperate at all. Furthermore, connecting and operating multiple PM motorson a single inverter that must function as the same synchronous speedspresent much more difficulty as variability in loading will causeinstabilities and finally loss of synchronism in the system.

BRIEF DESCRIPTION

In one aspect, a method of operating a drive circuit for parallelelectric motors, at least one of which is a PM motor, is provided. Themethod includes receiving measurements of stator phase currents in theparallel electric motors. The method includes selecting a target PMmotor, from among the parallel electric motors, that generates a largesttorque output. The method includes executing a vector control algorithmto generate a complex command voltage vector for the target PM motor.The method includes generating and transmitting a pulse width modulation(PWM) signal based on the complex command voltage vector for controllingan inverter. The method includes operating the inverter according to thePWM signal to supply three-phase alternating current (AC) power to theparallel electric motors.

In another aspect, a drive circuit for parallel electric motors, atleast one of which is a PM motor, is provided. The drive circuitincludes an inverter and a DSP. The inverter is coupled to the parallelelectric motors and is configured to supply an alternating current (AC)signal to stator windings thereof based on a PWM signal generated by theDSP. The DSP is coupled to the inverter and is configured to receiverespective stator phase current measurements for the parallel electricmotors. The DSP is further configured to select a target PM motor, ofthe parallel electric motors, having a largest torque output among theparallel electric motors. The DSP is further configured to execute avector control algorithm to generate a complex command voltage vectorfor the target PM motor. The DSP is further configured to generate thePWM signal based on the complex command voltage vector for the target PMmotor. The DSP is further configured to transmit the PWM signal to theinverter to operate the inverter and supply the AC signal to the statorwindings of the parallel electric motors.

In yet another aspect, a system is provided. The system includes a firstthree-phase motor, an inverter, an induction motor, a three-phase ACline-frequency bus, and a DSP. The first three-phase motor is configuredto drive a load. The inverter is coupled to the first three-phase motorand is configured to supply a three-phase output power thereto. Theinduction motor is configured to drive the load, and is furtherconfigured to be selectively coupled in parallel to the firstthree-phase motor and the inverter. The three-phase AC line-frequencybus is configured to be selectively coupled to the induction motor. TheDSP is coupled to the inverter and is configured to selectively, when ina direct operating mode, couple the induction motor to the three-phaseAC line-frequency bus, and decouple the induction motor from the firstthree-phase motor and the inverter. The DSP is further configured toselectively, when in a parallel operating mode, couple the inductionmotor to the inverter, and decouple the induction motor from thethree-phase AC line-frequency bus. The DSP is further configured tocontrol the inverter to supply the three-phase output power from theinverter to the first three-phase motor. The DSP is further configuredto control the inverter to supply the three-phase output power from theinverter to the induction motor when in the parallel operating mode.

In yet another aspect, a method of controlling fluid flow generated byfluid-moving equipment driven by a plurality of motors is provided. Themethod includes receiving a fluid flow demand signal indicating anamount of fluid flow commanded to be output by the fluid-movingequipment driven by the plurality of motors. The method includesdetermining a state of operation, of a plurality of predefined states ofoperation, in which to operate a drive circuit coupled to the pluralityof motors to generate the commanded amount of fluid flow. The methodincludes adjusting at least one relay associated with at least one ofthe plurality of motors to activate and selectively couple the at leastone motor to an inverter or an AC line-frequency power according to thedetermined state of operation. The method includes adjusting operationof the inverter to generate the commanded amount of fluid flow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an exemplary drive circuit for parallelmotors;

FIGS. 2A and 2B are graphs of exemplary complex command voltage vectorsfor parallel motors in a (d,q) coordinate system in a rotating rotorreference frame;

FIG. 3 is a functional block diagram of an exemplary drive circuit;

FIG. 4 is a flow diagram of an exemplary method of operating the drivecircuit shown in FIG. 3;

FIG. 5 is a flow diagram of an exemplary vector control method for usein the drive circuit shown in FIG. 3;

FIG. 6 is a schematic diagram of an exemplary drive circuit for use inthe drive circuit shown in FIGS. 1 and 3;

FIG. 7 is a schematic diagram of another exemplary drive circuit for usein the drive circuit shown in FIGS. 1 and 3;

FIG. 8 is a schematic diagram of another exemplary drive circuit for usein the drive circuit shown in FIGS. 1 and 3;

FIG. 9 is a schematic diagram of another exemplary drive circuit for usein the drive circuit shown in FIGS. 1 and 3;

FIG. 10 is a schematic diagram of another exemplary drive circuit foruse in the drive circuit shown in FIGS. 1 and 3;

FIG. 11 is a schematic diagram of an exemplary drive circuit for aparallel first motor and at least one induction motor;

FIG. 12 is a schematic diagram of an exemplary drive circuit for aparallel first induction motor and at least one additional inductionmotor;

FIG. 13 is a flow chart illustrating a fluid flow control method forcontrolling the drive circuits shown in FIG. 11 or FIG. 12; and

FIG. 14 is a schematic diagram of an exemplary drive circuit for aparallel first motor, induction motor, a third motor, and a fourthmotor.

DETAILED DESCRIPTION

As used herein, an element or step recited in the singular and precededwith the word “a” or “an” should be understood as not excluding pluralelements or steps, unless such exclusion is explicitly recited.Furthermore, references to “example implementation” or “oneimplementation” of the present disclosure are not intended to beinterpreted as excluding the existence of additional implementationsthat also incorporate the recited features.

It is realized herein that multiple electric motors may be operated by asingle inverter or drive device, including one or more PM motors orinduction motors in combination with one or more PM or induction motor.The drive circuits and methods of operation described herein provideparallel drive, using a single inverter, of two or more motors,operating under different load conditions and/or at different speeds.For example, parallel PM motors operate at a same speed, but may operateunder different load conditions. Embodiments of the drive circuitsdescribed herein provide estimation of rotor position and monitoring ofphase currents in parallel motors. Embodiments of the drive circuits andmethods control the inverter to operate each of the parallel motors, andsynchronize on the PM motor having the largest torque output, i.e., alargest load. In embodiments where only one PM motor is in parallel withone or more induction motor, the drive circuits and methods synchronizeon the one PM motor. Each other parallel motor, whether a PM motor or aninduction motor, is operated by the inverter using the same complexcommand voltage vector transformed into a PWM signal. In doing so, thedrive circuits and methods described herein dissipate excess current inthe stator windings of the other parallel motors in the form of anadditional flux current to resolve the complex command voltage vectorthat is the basis for the PWM signal that controls the single inverterfor all the parallel motors. The additional flux current enables each PMmotor to synchronize and stabilize when loading on the parallel PMmotors is imbalanced and torque outputs vary, while speed remains at thesynchronous speed, among the parallel PM motors. Further, in parallelinduction motors, the excess current in the stator windings isdissipated in the form of slip, because the induction motor operatesasynchronously. In certain embodiments, where an induction motor is inparallel with a PM or induction motor, the drive circuit selectivelyoperates the second induction motor “across the line,” i.e., powered bya line frequency AC signal and bypassing the inverter, or operated bythe inverter in parallel with the first motor to which the drive circuitsynchronizes. In such embodiments, the parallel motors have theflexibility to satisfy a variable load via the first motor with aparallel induction motor under partial loading, the first motor incombination with an induction motor under partial-to-full loading, orthe first motor alone for full-to-low loading.

It is further realized herein, in embodiments of the drive circuit, thesingle inverter that operates parallel motors should be controlled by aDSP, or other suitable processor, executing, for example, a vectorcontrol algorithm that produces a complex command voltage vector onwhich a PWM signal is based for controlling each phase of the inverter.The DSP and inverter, in certain embodiments, may be co-located withinthe drive circuit. In other embodiments, the DSP is remote from theinverter. In certain embodiments, the drive circuit, including the DSPand/or inverter, is disposed centrally with respect to the multipleparallel motors. In alternative embodiments, the drive circuit may beincorporated, for example, into the package of one of the parallel PMmotors.

FIG. 1 is a block diagram of an exemplary drive circuit 100 for parallelmotors 102 and 104, at least one of which is a PM motor. Each of motors102 and 104 includes a rotor (not shown) and a stator, including statorwindings 106. Notably, motors 102 and 104 are coupled in parallel todrive circuit 100. Drive circuit 100 includes a DSP 108, an inverter110, and current sensors 112.

Current sensors 112 include any device that is operable to produce asignal that represents current amplitude. For example, current sensors112 may include shunt sensing resistors, current transformers,hall-effect current measurement integrated circuits, or any othersuitable device for measuring current.

DSP 108 executes a control algorithm, such as, for example, a vectorcontrol algorithm, for controlling inverter 110. More specifically, DSP108 transmits one or more PWM signals 114 to inverter 110 to control theoperation of various switches and power electronics (not shown) withininverter 110. Inverter 110, during operation, converts an input power116, such as, for example, a DC power or an AC rectified power, tothree-phase power for energizing stator windings 106 of motors 102 and104. In such an embodiment, DSP 108 may transmit a PWM signal 114 foreach phase of inverter 110 to generate the three phases of output power(W, U, V). DSP 108 generates a given PWM signal 114 based on statorcurrent measurements collected by current sensors 112. Stator currentmeasurements for each phase of each motor may be determined based oncurrent sensors couple to various portions of drive circuit 100,including, for example, within inverter 110, individual phase legmeasurements 118, collective measurements 120 at the output of inverter110, or any combination thereof.

DSP 108 is further configured to generate a given PWM signal 114 basedon rotor position of one or more of motors 102 and 104. Drive circuit100 receives rotor speed measurements 122 and 124 from motors 102 and104, respectively. Rotor speed measurements 122 and 124 may beintegrated over time to determine a rotor position. Rotor speed may bemeasured by respective sensors (not shown) coupled to motors 102 and104. In certain embodiments, rotor speed is derived from the outputfrequency of three-phase power (W, U, V) of inverter 110. In alternativeembodiments, rotor position is measured directly. In other embodiments,DSP 108 executes a position-sensorless vector control algorithm androtor speeds measurements 122 and 124 are not used.

FIGS. 2A and 2B are graphs of exemplary complex command voltage vectorsfor parallel motors, at least one of which is a PM motor, such as motors102 and 104 shown in FIG. 1, in a (d,q) coordinate system 200 in arotating rotor reference frame. (d,q) coordinate system 200 is definedby a direct (d) axis 202 representing flux and a quadrature (q) axis 204representing torque, the positions of which are determined as a functionof the rotor angle derived from the rotor flux linkage vector estimatedaccording to the rotor position and stator current.

FIG. 2A illustrates a command voltage vector 206 for a first PM motorhaving the greatest torque output among the parallel PM motors.Accordingly, command voltage vector 206 is illustrated as a vector sumof a torque component 208 and a flux component 210. Torque component 208is a sum of a back electromagnetic force (EMF) voltage vector 212 (E)and the product of a torque current vector 214 (I) and scaled by aresistance 216 (R), and projected onto the q axis. The magnitude of fluxcomponent 210 of command voltage vector 206 is computed as:

−P·L_(q)·ω·I_(q1),

where P is a quantity of pole-pairs in the motor, L_(q) is statorwinding inductance, ω is the frequency at which the synchronous motorturns, and I_(q1) is the torque current.

FIG. 2B illustrates command voltage vector 206 applied to a second motorin parallel with the first PM motor represented in FIG. 2A. Commandvoltage vector 206 likewise has a torque component 218 and a fluxcomponent 220, both of which are of lesser magnitude than torquecomponent 208 and flux component 210 shown in FIG. 2A, given that eachis at least partially determined as a function of torque output, whichis less in the second motor. Torque component 218, as in FIG. 2A, is asum of a back EMF voltage vector 222 (E) and the product of a torquecurrent vector 224 (I) and scaled by a resistance 226 (R), and projectedonto the q axis. Given that the first PM motor has the greatest torqueoutput, torque current vector 224 must have a lesser magnitude thantorque current vector 214 shown in FIG. 2A. Resistance scalar 216 andresistance scalar 226 are assumed substantially equal, as are back EMFvoltage vectors 212 and 222 for equivalent motors. In certainembodiments, where the parallel PM motors are not equivalent machines,the resistance vectors and back EMF vectors would vary, but aregenerally known for a given motor.

Command voltage vector 206, when applied to the second motor representedin FIG. 2B, delivers more current than is necessary to the statorwindings to produce the demanded torque output, i.e., the torqueproducing current, I_(q2), is less than in the first PM motor.Consequently, the excess current is dissipated by the second motor inthe form of an additional flux current 228, which mathematicallysatisfies the vector control algorithm.

FIG. 3 is a functional block diagram of an exemplary drive circuit 300for operating parallel motors 302, 304, and 306, including at least onePM motor 302. Motors 304 and 306 may each be parallel PM motors orinduction motors. In the embodiment of FIG. 3, motors 302, 304, and 306are three-phase motors. Drive circuit 300 operates at least two motorsin parallel. In alternative embodiments, for example, drive circuit 300operates two parallel motors. In other embodiments, drive circuit 300operates four or more parallel motors. In the embodiment of FIG. 3,drive circuit 300 operates three motors in parallel. In certainembodiments, one or more of the parallel motors may be selectivelyoperated by drive circuit 300 or, when the parallel motors are inductionmotors, may be operated directly across line frequency power from, forexample, a utility or an AC generator.

Drive circuit 300 includes a DSP 308 coupled to an inverter 310.Inverter 310 converts an input power 312 to three-phase AC power 314that is supplied to the stator windings of motors 302, 304, and 306. Inthe embodiment of FIG. 3, input power 312 is DC power. In alternativeembodiments, input power 312 may include an AC power supply and inverter310 is combined with a rectifier (not shown) to transform the input ACpower to an appropriate three-phase AC power 314 to be supplied tomotors 302, 304, and 306. Inverter 310 is controlled by one or more PWMsignals 316 generated by and transmitted from DSP 308. For example, whenoperating three-phase motors, DSP 308 may transmit a PWM signal 316 forindependently controlling each phase leg in inverter 310.

DSP 308 controls inverter 310 based at least in part on stator currents318 measured or otherwise determined in each of motors 302, 304, and306. Stator currents 318 are time varying signals indicating at leastthe amplitude and phase of the current supplied to each phase of therespective stators of motors 302, 304, and 306. DSP 308 may furthercontrol inverter 310 based on respective positions and/or speeds of therotors of motors 302, 304, and 306 as measured or otherwise determined.For example, in certain embodiments, PM motor speed, i.e., the speed atwhich the rotor turns, is monitored, as it is approximately the same asthe frequency of the current supplied to the stator windings. The PMmotor speed may then be integrated over time to determine its rotorposition. In other embodiments, a position sensorless vector controlalgorithm may be used to estimate the rotor speeds and positions withoutthe use of a position sensor.

DSP 308 includes several functional modules for controlling inverter310. Namely, DSP 308 includes a motor selection module 320, a vectorcontrol algorithm 322, and a PWM signal generator 324. Each of thesemodules, among others, are implemented in software or firmware, orotherwise programmed onto DSP 308 to be executed by DSP 308 for carryingout their respective functions.

Motor selection module 320 determines which of motors 302, 304, and 306should be the target of synchronizing using vector control algorithm322. Although PM motor 302 is indicated as the target PM motor, motorselection module 320 may select any of motors 302, 304, and 306 based ontheir respective torque outputs. More specifically, motor selectionmodule 320 determines which of motors 302, 304, and 306 is producing thegreatest torque output, and selects that motor, e.g., PM motor 302, tobe the target of synchronizing using vector control algorithm 322. Incertain embodiments, motor selection module 320 makes this determinationbased on stator currents 318. For example, motor selection module 320receives stator currents 318 for each of motors 302, 304, and 306. Motorselection module 320 then determines which of motors 302, 304, and 306is drawing the greatest torque producing current through its respectivestator windings. The current conducted through the stator windingscorrelates to the torque output of a given motor. Accordingly, the motordrawing the greatest torque producing current correlates to the motorgenerating the greatest torque output.

As noted above, at least one of motors 302, 304, and 306 is a PM motor.Motor selection module 320 only selects a PM motor from among theparallel motors to be the target of synchronization by vector controlalgorithm 322. In embodiments where one or both of motors 304 and 306are induction motors, an induction motor is not selected by motorselection module 320, because such an induction motor operatesasynchronously and is not amenable to synchronization by vector controlalgorithm 322.

In the embodiment of FIG. 3, motor selection module 320 determines PMmotor 302 is generating the greatest torque output among motors 302,304, and 306. DSP 308 then executes vector control algorithm 322 withrespect to target PM motor 302. Vector control algorithm 322 generates acomplex command voltage vector, such as, for example, command voltagevector 206 shown in FIG. 2A, that will be the basis for PWM signals 316generated by PWM signal generator 324. Vector control algorithm 322computes the command voltage vector based at least on stator currents318 for the selected motor, e.g., PM motor 302. Vector control algorithmcomputes the complex command voltage vector, expressed in a rotatingrotor reference frame and in the (d,q) coordinate system, to provide anefficient balance of torque current and flux current to the statorwindings (not shown) of PM motor 302. Torque current and flux currenteach result in a respective voltage drop. These voltage drops areillustrated, for example, as torque component 208 and flux component 210shown in FIG. 2A. The torque current results in a voltage drop along thed-axis, e.g., the flux component, and the flux current results in avoltage drop along the q-axis, e.g., the torque component 208. Vectorcontrol algorithm 322 includes a scalar control portion to independentlycompute these voltage drops, which are then translated to the complexcommand voltage vector that is transmitted to PWM signal generator 324.

PWM signal generator 324 generates one or more PWM signals forcontrolling inverter 310. For example, in one embodiment, PWM signalgenerator 324 generates a PWM signal 316 for controlling each phase legwithin inverter 310. Given that inverter 310 provides the samethree-phase AC power 314 to each of motors 302, 304, and 306, each motorof motors 302, 304, and 306 is thereby controlled by the same complexcommand voltage vector that results from vector control algorithm 322executing on the targeted PM motor 302. Consequently, as shown in FIG.2B, excess current in the stator windings of motors 304 and 306 isdissipated in the form of additional flux current 228.

FIG. 4 is a flow diagram of an exemplary method 400 of operating drivecircuit 300, shown in FIG. 3, for operating parallel electric motors, atleast one of which is a PM motor. DSP 308 receives 410 stator currents318 measured or otherwise determined for the respective phases of eachof the parallel electric motors 302, 304, and 306. DSP 308 selects 420target PM motor 302 from among the parallel electric motors 302, 304,and 306. DSP 308 selects 420 PM motor 302 based on its generating alargest torque output. DSP 308 executes 430 vector control algorithm 322to generate a complex command voltage vector, such as command voltagevector 206 shown in FIG. 2A, for target PM motor 302. DSP 308 thengenerates and transmits 440 PWM signal 316 to inverter 310 based on thecomplex command voltage vector for controlling inverter 310. DSP 308then operates 450 inverter 310 according to PWM signal 316 to supplythree-phase AC power 314 to parallel electric motors 302, 304, and 306.

FIG. 5 is a flow diagram of an exemplary vector control method 500, suchas, for example, vector control algorithm 322, for use in drive circuit300, both shown in FIG. 3. Vector control method 500 is generallyimplemented within DSP 308 and begins with determining 510 a rotorposition for target PM motor 302 based on a directly measured positionor, in certain embodiments, from a position sensorless rotor angleestimator. Given knowledge of the rotor angle, a flux-torque (d,q)coordinate system is defined in which flux and torque component vectorsare calculated. The respective stator phase currents for target PM motor302 are then transformed 530 into the flux-torque coordinate system. DSP308 then computes 540 commanded flux and torque voltage componentsusing, for example, PI control methods, based on the stator phasecurrents for the target PM motor. The complex command voltage vector isthen computed 550 based on the computed commanded flux and torquevoltage components.

FIG. 6 is a schematic diagram of an exemplary drive circuit 600 for aplurality of parallel electric motors, for example, a first motor 602and a second motor 604, up to an nth motor 605. Motors 602, 604 . . .605 may be PM motors, induction motors, or any combination thereof. Incertain embodiments, at least one of motors 602, 604 . . . 605 is a PMmotor. Each of motors 602, 604 . . . 605 includes a rotor (not shown)and a stator, including stator windings 606. Notably, motors 602, 604 .. . 605 are coupled in parallel to drive circuit 600. Drive circuit 600includes a DSP 608, an inverter 610, and at least one current sensor612. Current sensor 612 includes any device that is operable to producea signal that represents current amplitude. For example, current sensors612 may include shunt sensing resistors, current transformers,hall-effect current measurement integrated circuits, or any othersuitable device for measuring current.

In the exemplary embodiment, inverter 610 is configured to be coupled toand provide three phase power to a plurality of parallel electric motors602, 604 . . . 605. Inverter 610 is a three-phase voltage sourceinverter that is configured to convert input power 616 to three-phasepower for energizing stator windings 606 of parallel motors 602, 604 . .. 605 based on control signals received from DSP 608.

In the exemplary embodiment, inverter 610 includes a first phase leg614, a second phase leg 616, and a third phase leg 618. First, second,and third phase legs 614, 616, and 618 include a DC input stage 620 andan AC output stage 622. DC input stage 620 provides input power 116 tofirst, second, and third phase legs 614, 616, and 618 via a positive DClink rail 624 and a negative DC link rail 626. AC output stage 622facilitates outputting stator phase currents from first, second, andthird phase legs 614, 616, and 618 of inverter 610 to stator windings606 of motors 602, 604 . . . 605.

More specifically, first phase leg 614 includes a first switch 628 and asecond switch 630 serially-coupled between positive and negative DC linkrails 624 and 626. A first output node 627 is defined between first andsecond switches 628 and 630, and is configured to be electricallyconnected to and provide a first stator phase current to a first-phasestator winding 606 of each motor 602, 604 . . . 605.

Additionally, second phase leg 616 includes a third switch 632 and afourth switch 634 serially-coupled between positive and negative DC linkrails 624 and 626. A second output node 635 is defined between third andfourth switches 632 and 634, and is configured to be electricallyconnected to and provide a second phase current to a second-phase statorwinding 606 of each of motors 602, 604 . . . 605.

Moreover, third phase leg 618 includes a fifth switch 636 and a sixthswitch 638 serially-coupled between positive and negative DC link rails624 and 626. A third output node 639 is defined between fifth and sixthswitches 636 and 638, and is configured to be electrically connected toand provide a third phase current to a third-phase stator winding 606 ofeach of motors 602, 604 . . . 605.

Drive circuit 600 includes at least one current sensor 612 coupled toinverter 610 and configured to measure stator phase currents output byinverter 610 for driving the plurality of parallel electric motors 602,604 . . . 605. In the exemplary embodiment, current sensor 612 is ashunt resistor 640 coupled to DC input stage 620 and is configured tomeasure total current at DC input stage 620. More specifically, shuntresistor 640 is coupled in-line to negative DC link rail 626. Shuntresistor 640 measures DC link current of input power 616 along negativeDC link rail 626.

DSP 608 receives the total current at the DC input stage 620 from shuntresistor 640. DSP 608 then determines electrical routes taken bycurrents flowing through inverter 610 based on known switching sequencesof inverter 610. DSP 608 reconstructs stator phase currents for theplurality of parallel electric motors 602, 604 . . . 605 by correlatingthe determined electrical routes to total currents from shunt resistor640 received over a time period. Each reconstructed stator phase currentrepresents a total current on each particular phase from all of motors602, 604 . . . 605. DSP 608 may determine average phase currents acrossmotors 602, 604 . . . 605 by dividing the stator phase currents by thetotal number of paralleled motors 602, 604 . . . 605. DSP 608 thengenerates at least one PWM signal for controlling inverter 610 based onthe reconstructed stator phase currents.

In an alternative embodiment, the at least one current sensor 612includes at least two shunt resistors respectively coupled to two phaselegs of first, second, and third phase legs 614, 616, and 618. Forexample, the at least two shunt resistors may include a first shuntresistor 642 coupled to first phase leg 614 and a second shunt resistor644 coupled to second phase leg 616. However, this selection isdescribed for exemplary purposes only and it should be understood thatfirst and second shunt resistors 642 and 644 may be coupled with anyselection to any of first, second, and third phase legs 614, 616, and618. First shunt resistor 642 is coupled to first phase leg 614 at theDC input stage 620 and is configured to measure a stator phase currentfor first phase leg 614. The stator phase current for first phase leg614 represents a sum of first phase currents for each of the pluralityof parallel electric motors 602, 604 . . . 605. Additionally, secondshunt resistor 644 is coupled to second phase leg 616 at the DC inputstage 620. Second shunt resistor 644 is configured to measure a statorphase current for second phase leg 616. The stator phase current forsecond phase leg 616 represents a sum of second phase currents for eachof the plurality of parallel electric motors 602, 604 . . . 605.

DSP 608 receives the stator phase currents for first phase leg 614 andsecond phase leg 616 from first and second shunt resistors 642 and 644,respectively. Because each of the motors 602, 604 . . . 605 must havethe sum of its phase currents equal to 0, DSP 608 can estimate a statorphase current for third phase leg 618 based on the stator phase currentsreceived from first and second shunt resistors 642 and 644. DSP 608 isfurther configured to divide the stator phase currents for each offirst, second, and third phase legs 614, 616, and 618 by a total numberof the plurality of parallel electric motors 602, 604 . . . 605 todetermine average stator phase currents in each of the plurality ofparallel electric motors 602, 604 . . . 605. Based on the average statorphase currents, DSP 608 generates at least one PWM signal forcontrolling inverter 610.

Additionally, in one embodiment, the at least two shunt resistorsfurther include a third shunt resistor 646 coupled to third phase leg618 at the DC input stage 620. Third shunt resistor 646 is configured tomeasure a stator phase current for third phase leg 618. The stator phasecurrent for third phase leg 618 represents a sum of third phase currentsfor each of the plurality of parallel electric motors 602, 604 . . .605.

First, second, and/or third shunt resistors 642, 644, and 646 arecommunicatively coupled to DSP 608 and provide the respective measuredstator currents to DSP 608. Based on the measured stator currents, DSP608 generates a respective PWM signal 114 for each phase of inverter 610to generate the three phases of output power (W, U, V) for applicationto motors 602, 604 . . . 605.

FIG. 7 is a schematic diagram of an exemplary drive circuit 700 for aplurality of parallel electric motors 602, 604 . . . 605. Drive circuit700 is similar to drive circuit 600 (shown in FIG. 6), except drivecircuit 700 includes a second current sensor coupled to the AC outputstage of the first phase leg for every parallel electric motor in excessof a first electric motor coupled to drive circuit 700. As such,components shown in FIG. 7 that are substantially similar to componentsshown in FIG. 6 are labeled with the same reference numbers used in FIG.6.

In the exemplary embodiment, for every additional motor (i.e., motors604 . . . 605) coupled to drive circuit 700 in excess of first motor602, a second current sensor 702 is coupled in-line between first outputnode 627 and a first stator phase winding 704 of each parallel electricmotor 604 . . . 605 in excess of the first electric motor 602. Eachsecond current sensor 702 is configured to measure a first stator phasecurrent in a respective parallel electric motor 604 . . . 605 in excessof first motor 602.

A DSP 708 is configured to receive the measured first stator phasecurrent of second motor 604 from second current sensor 702. Usingsynchronous motor principles and based on the first stator phase currentfrom second current sensor 702, DSP 708 estimates a second stator phasecurrent and a third stator phase current for motor 604 that is in excessof first motor 602. DSP 708 may repeat this process of determining thephase currents for any additional parallel motors.

Additionally, and as described above with reference to FIG. 6, DSP 708determines total phase currents output by inverter 610 to all parallelmotors 602, 604 . . . 605 using one of shunt resistor 640, first andsecond shunt resistors 642 and 644, or first, second, and third shuntresistors 642, 644, and 646. DSP 708 is configured to determinedifferences between the first, second, and third stator phase currentsof parallel motor 604 in excess of first motor 602 and respective totalstator phase currents output from first, second, and third phase legs614, 616, and 618. First, second, and third stator phase currents of thefirst electric motor 602 are reconstructed based on the determineddifferences. After having determined each stator phase current for eachparallel motor, DSP 708 generates at least one PWM signal forcontrolling inverter 610 based on the stator phase currents.

FIG. 8 is a schematic diagram of an exemplary drive circuit 800 for aplurality of parallel electric motors 602, 604 . . . 605. Drive circuit800 is similar to drive circuit 600 (shown in FIG. 6), except drivecircuit 800 includes an additional phase leg for each parallel electricmotor in excess of a first electric motor coupled to drive circuit 800.As such, components shown in FIG. 8 that are substantially similar tocomponents shown in FIG. 6 are labeled with the same reference numbersused in FIG. 6.

In the exemplary embodiment, and as shown in FIG. 8, drive circuit 800includes additional motor 604 that is in excess of first motor 602, soan additional leg, for example, a first additional phase leg 802, isprovided within inverter 610. First additional phase leg 802 includes DCinput stage 620 and AC output stage 622, wherein AC output stage 622 offirst additional phase leg 802 is configured to be coupled to a firststator phase winding 804 of a respective parallel electric motor 604 inexcess of first electric motor 602.

First additional leg 802 includes a seventh switch 806 and an eighthswitch 809 serially-coupled between positive and negative power busrails 624 and 626. First additional leg 802 also includes a fourthoutput node 810 defined between seventh and eighth switches 806 and 809.Fourth output node 810 is configured to be electrically connected to andprovide a first phase current to first-phase stator winding 804 of motor604. Although described herein as being coupleable to a first-phasewinding, it is to be understood that the one or more additional legs mayinstead be configured for coupling to the second phase or the thirdphase winding connections of motors 604 . . . 605.

In the exemplary embodiment, a second current sensor 812 is coupled tofirst additional leg 802 at DC input stage 620. Second current sensor812 is configured to measure a first stator phase current of firstadditional phase leg 802.

Drive circuit 800 provides two common stator phase currents (i.e.,second and third phases) for all motors 602, 604 . . . 605. DSP 808 isconfigured to independently control first stator phase currents of firstelectric motor 602 and additional parallel-coupled motor 604 in excessof first motor 602.

Adding an additional phase leg for each additional motor coupled todrive circuit 800 provides an additional degree of freedom per motor,which significantly expands the control capabilities of DSP 808. Forexample, a minor adjustment of the first phase current would result inan adjustment in the total production of all motors 602, 604 . . . 605,making it easier to maintain stability. Moreover, if there is animbalance in one of motors 602, 604 . . . 605, DSP 808 may adjust thefirst phase current within the imbalanced motor. Further, the firstphase currents of any additional phase legs may be sensed and used byDSP 808 to monitor motors 604 . . . 605 for proper operation.

For any additional motor(s) parallel-coupled to drive circuit 800,another additional leg is added to inverter 610, the output of which iscoupled to first-phase stator winding 804 of the additional motor(s).For example, if nth motor 605 is parallel-coupled to drive circuit 800,a second additional phase leg 814 would be provided in inverter 610.Second additional phase leg 814 includes similar components as andoperates substantially similarly to first additional phase leg 802, sothe details will not be repeated herein. Also, a third current sensor816 that functions substantially similarly to second current sensor 812is coupled to second additional phase leg 814 for measuring a firststator phase current of second additional phase leg 814.

FIG. 9 is a schematic diagram of an exemplary drive circuit 900 for aplurality of parallel electric motors, for example, a first motor 902and a second motor 904, up to an nth motor 905. Motors 902, 904 . . .905 may be PM motors, induction motors, or any combination thereof. Incertain embodiments, at least one of motors 902, 904 . . . 905 is a PMmotor. Each of motors 902, 904 . . . 905 includes a rotor (not shown)and a stator, including stator windings 906. Notably, motors 902, 904 .. . 905 are coupled in parallel to drive circuit 900. Drive circuit 900includes a DSP 908, an inverter 910, and at least two current sensors912.

In the exemplary embodiment, inverter 910 is configured to be coupled toand provide three phase power to a plurality of parallel electric motors902, 904 . . . 905. Inverter 910 is a three-phase voltage sourceinverter that is configured to convert input power 916 to three-phasepower for energizing stator windings 906 of parallel motors 902, 904 . .. 905 based on control signals received from DSP 908. In the exemplaryembodiment, at least one of motors 902, 904 . . . 905 is a PM motor,while the remaining motors 902, 904 . . . 905 may include PM motorsand/or induction motors.

In the exemplary embodiment, inverter 910 includes a first phase leg914, a second phase leg 916, and a third phase leg 918. First, second,and third phase legs 914, 916, and 918 include a DC input stage 920 andan AC output stage 922. DC input stage 920 provides input power 116 tofirst, second, and third phase legs 914, 916, and 918 via a positive DClink rail 924 and a negative DC link rail 926. AC output stage 922facilitates outputting stator phase currents from first, second, andthird phase legs 914, 916, and 918 of inverter 910 to stator windings906 of motors 902, 904 . . . 905.

More specifically, first phase leg 914 includes a first switch 928 and asecond switch 930 serially-coupled between positive and negative DC linkrails 924 and 926. A first output node 927 is defined between first andsecond switches 928 and 930, and is configured to be electricallyconnected to and provide a first stator phase current to a first statorphase winding 907 of each motor 902, 904 . . . 905.

Additionally, second phase leg 916 includes a third switch 932 and afourth switch 934 serially-coupled between positive and negative DC linkrails 924 and 926. A second output node 935 is defined between third andfourth switches 932 and 934, and is configured to be electricallyconnected to and provide a second phase current to a second stator phasewinding 909 of each of motors 902, 904 . . . 905.

Moreover, third phase leg 918 includes a fifth switch 936 and a sixthswitch 938 serially-coupled between positive and negative DC link rails924 and 926. A third output node 939 is defined between fifth and sixthswitches 936 and 938, and is configured to be electrically connected toand provide a third phase current to a third stator phase winding 911 ofeach of motors 902, 904 . . . 905.

In the exemplary embodiment, at least two current sensors 912 arecoupled to inverter 910 and are configured to measure stator phasecurrents output by inverter 910 for driving the plurality of parallelelectric motors 902, 904 . . . 905. More specifically, the at least twocurrent sensors 912 are respectively coupled to two phase legs of first,second, and third phase legs 914, 916, and 918. For example, the atleast two current sensors 912 may include a first current sensor 942coupled to first phase leg 914 and a second current sensor 944 coupledto second phase leg 916. However, this orientation is described forexemplary purposes only and it should be understood that first andsecond current sensors 942 and 944 may be coupled with any orientationto any of first, second, and third phase legs 914, 916, and 918. Firstcurrent sensor 942 is coupled between the AC output stage 922 of firstphase leg 914 and commonly-coupled first stator phase windings 907 ofmotors 902, 904 . . . 905. First current sensor 942 is configured tomeasure a stator phase current for first phase leg 914. The stator phasecurrent for first phase leg 914 represents a sum of first phase currentsfor each of the plurality of parallel electric motors 902, 904 . . .905.

Additionally, second current sensor 944 is coupled between the AC outputstage 922 of second phase leg 916 and commonly-coupled second statorphase windings 909 of motors 902, 904 . . . 905. Second current sensor944 is configured to measure a stator phase current for second phase leg916. The stator phase current for second phase leg 916 represents a sumof second phase currents for each of the plurality of parallel electricmotors 902, 904 . . . 905.

DSP 908 receives the stator phase currents for first phase leg 914 andsecond phase leg 916 from first and second current sensors 942 and 944,respectively. Because the motors 902, 904 . . . 905 are synchronous, DSP908 can estimate a stator phase current for third phase leg 918 based onthe stator phase currents received from first and second current sensors942 and 944. DSP 908 is further configured to divide the stator phasecurrents for each of first, second, and third phase legs 914, 916, and918 by a total number of the plurality of parallel electric motors 902,904 . . . 905 to determine average stator phase currents in each of theplurality of parallel electric motors 902, 904 . . . 905. Based on theaverage stator phase currents, DSP 908 generates at least one PWM signalfor controlling inverter 910.

Additionally, in one embodiment, the at least two current sensors 912further include a third current sensor 946 coupled between the AC outputstage 922 of third phase leg 918 and commonly-coupled third stator phasewindings 911 of motors 902, 904 . . . 905. Third current sensor 946 isconfigured to measure a stator phase current for third phase leg 918.The stator phase current for third phase leg 918 represents a sum ofthird phase currents for each of the plurality of parallel electricmotors 902, 904 . . . 905.

FIG. 10 is a schematic diagram of an exemplary drive circuit 1000 for aplurality of parallel electric motors, for example, a first motor 1002and a second motor 1004, up to an nth motor 1005. Motors 1002, 1004 . .. 1005 may be PM motors, induction motors, or any combination thereof.In certain embodiments, at least one of parallel motors 1002, 1004 . . .1005 is a permanent magnet (PM) motor. Each of motors 1002, 1004 . . .1005 includes a rotor (not shown) and a stator, including statorwindings 1006. Notably, motors 1002, 1004 . . . 1005 are coupled inparallel to drive circuit 1000. Drive circuit 1000 includes a DSP 1008,an inverter 1010, and at least one current sensor 1012.

In the exemplary embodiment, inverter 1010 is configured to be coupledto and provide three phase power to a plurality of parallel electricmotors 1002, 1004 . . . 1005. Inverter 1010 is a three-phase voltagesource inverter that is configured to convert input power 1016 tothree-phase power for energizing stator windings 1006 of parallel motors1002, 1004 . . . 1005 based on control signals received from DSP 608. Inthe exemplary embodiment, at least one of motors 1002, 1004 . . . 1005is a PM motor, while the remaining motors 1002, 1004 . . . 1005 mayinclude PM motors and/or induction motors.

In the exemplary embodiment, inverter 1010 includes a first phase leg1014, a second phase leg 1016, and a third phase leg 1018. First,second, and third phase legs 1014, 1016, and 1018 include a DC inputstage 1020 and an AC output stage 1022. DC input stage 1020 providesinput power 116 to first, second, and third phase legs 1014, 1016, and1018 via a positive DC link rail 1024 and a negative DC link rail 1026.AC output stage 1022 facilitates outputting stator phase currents fromfirst, second, and third phase legs 1014, 1016, and 1018 of inverter1010 to stator windings 1006 of motors 1002, 1004 . . . 1005.

More specifically, first phase leg 1014 includes a first switch 1028 anda second switch 1030 serially-coupled between positive and negative DClink rails 1024 and 1026. A first output node 1027 is defined betweenfirst and second switches 1028 and 1030, and is configured to beelectrically connected to and provide a first stator phase current to afirst-phase stator winding 1006 of each motor 1002, 1004 . . . 1005.

Additionally, second phase leg 1016 includes a third switch 1032 and afourth switch 1034 serially-coupled between positive and negative DClink rails 1024 and 1026. A second output node 1035 is defined betweenthird and fourth switches 1032 and 1034, and is configured to beelectrically connected to and provide a second phase current to asecond-phase stator winding 1006 of each of motors 1002, 1004 . . .1005.

Moreover, third phase leg 1018 includes a fifth switch 1036 and a sixthswitch 1038 serially-coupled between positive and negative DC link rails1024 and 1026. A third output node 1039 is defined between fifth andsixth switches 1036 and 1038, and is configured to be electricallyconnected to and provide a third phase current to a third-phase statorwinding 1006 of each of motors 1002, 1004 . . . 1005.

In the exemplary embodiment, drive circuit 1000 includes an independentcurrent sensor 1012 coupled in-line with at least one phase leg outputof each paralleled motor 1002, 1004 . . . 1005. For example, for motor1002, drive circuit 1000 includes current sensor 1012 for measuringcurrent of at least one phase of motor 1002. More specifically, in theexemplary embodiment, current sensor 1012 is coupled between firstoutput node 1027 and first phase stator winding 1007 of motor 1002.Additionally, or alternatively, drive circuit 1000 includes anadditional current sensor 1012 coupled between second output node 1035and second phase stator winding 1009 of motor 1002. Additionally, oralternatively, drive circuit 1000 further includes a third currentsensor 1012 coupled between third output node 1039 and third phasestator winding 1011 of motor 1002.

Moreover, in the exemplary embodiment, for motor 1004, drive circuit1000 includes current sensor 1012 for measuring current of at least onephase of motor 1004. More specifically, in the exemplary embodiment,current sensor 1012 is coupled between first output node 1027 and firstphase stator winding 1007 of motor 1004. Additionally, or alternatively,drive circuit 1000 includes an additional current sensor 1012 coupledbetween second output node 1035 and second phase stator winding 1009 ofmotor 1004. Additionally, or alternatively, drive circuit 1000 furtherincludes a third current sensor 1012 coupled between third output node1039 and third phase stator winding 1011 of motor 1004.

In the embodiments where only one current sensor 1012 is used for eachof motors 1002 and 1004, one or more current reconstruction algorithmsare implemented by DSP 1008 to reconstruct the other two phase currentsfor each of motor 1002 and 1004. Where two current sensors 1012 are usedfor each of motors 1002 and 1004, because the motors are synchronous,DSP 1008 can estimate the third phase currents for motors 1002 and 1004based on the two measured stator phase currents. Finally, where currentsensors 1012 are used on all three phases of each motor 1002 and 1004,every stator phase current is measured for each motor 1002 and 1004,and, accordingly, DSP 1008 does not have to perform phase currentestimation/calculation and/or current reconstruction.

Having independent current sensing on one or more phases of eachparallel motor provides DSP 1008 with greater control over motors 1002,1004 . . . 1005. For example, phase angles between phase currents may beselectively adjusted to achieve synchronous operation of the motors.Further, measured stator currents may be compared to one another toconfirm proper motor operation.

FIG. 11 is a schematic diagram of an exemplary drive circuit 1100 for afirst motor 1102 in parallel with an induction motor 1104. In theexemplary embodiment, first motor 1102 may be either a PM motor or aninduction motor. Drive circuit 1100 includes a DSP 1106 coupled to andconfigured to control an inverter 1108 and relays R1 and R2. In someembodiments, the states of relays R1 and R2 may be commanded by a systemcontroller that also transmits a command signal to DSP 1106. In otherembodiments, control of relays R1 and R2 may be shared between DSP 1106and the system controller.

DSP 1106 determines how to operate first motor 1102 and induction motor1104 based on a detected load 1112 on the motors. DSP 1106, in certainembodiments, may quantify load 1112 based on measurements external todrive circuit 1100, such as, for example, ambient air temperature in anHVAC system. In alternative embodiments, DSP 1106 may quantify load 1112based on power output to first motor 1102 and induction motor 1104, suchas, for example, monitoring current delivered to first motor 1102 andinduction motor 1104.

Drive circuit 1100 enables operation of first motor 1102 and inductionmotor 1104 in one of two modes, and further enables transition amongthose modes. In the exemplary embodiment, first motor 1102 is coupled toand is driven using variable speed by inverter 1108 or may be driven bythe power source when the inverter is in the off state. Induction motor1104 may be driven in parallel with first motor 1102 through inverter1108, or directly across a three-phase AC line-frequency power 1114,which may be supplied, for example, by a three-phase generator or autility. Operating induction motor 1104 directly across three-phase ACline-frequency power 1114 enables the greatest power output frominduction motor 1104 and is selected by DSP 1106 under full loading,e.g., load 1112 is at its greatest. Operating induction motor 1104through inverter 1108 enables induction motor 1104 to operate at a lowertorque output to meet partial loads.

DSP 1106 controls inverter 1108 to operate first motor 1102 as describedabove with respect to drive circuit 300, shown in FIG. 3. Morespecifically, in the exemplary embodiment, DSP 1106 uses vector control,such as vector control algorithm 322 shown in FIG. 3 or method 500 shownin FIG. 5, to compute a complex command voltage vector that is the basisfor a PWM signal 1116 that is transmitted to inverter 1108 to controlinverter 1108. In other embodiments, DSP 1106 may use scalar control forinduction motor 1104, or any other known technique commonly used to varyspeed of induction motors. Inverter 1108, based on PWM signal 1116,converts a DC input power 1118 into a three-phase output power 1120 thatis supplied to the stator windings (not shown) of first motor 1102. Incertain embodiments, drive circuit 1100 may operate one or moreadditional PM or induction motors in parallel with first motor 1102 andusing DSP 1106 and inverter 1108. In such embodiments where motors arein parallel with a PM motor, as described above with respect to drivecircuit 300, DSP 1106 determines which of the parallel PM motors isgenerating the greatest torque output, and selects that PM motor as thetarget PM motor on which vector control operates. DSP 1106 then controlsinverter 1108 to operate each of the parallel PM motors using the samecomplex command voltage vector and corresponding PWM signal.

DSP 1106 controls the configuration of relays R1 and R2 to select a modeof operation of first motor 1102 and induction motor 1104. Relays R1 andR2 may be any suitable power switching devices suitable for coupling anddecoupling AC power sources, such as three-phase AC line-frequency power1114 or inverter 1108, to first motor 1102 and induction motor 1104.

DSP 1106 operates first motor 1102 and induction motor 1104 in parallelby closing relay R2 and opening relay R1. Relay R1 operates to isolateinverter 1108, first motor 1102, and induction motor 1104 fromthree-phase AC line-frequency power 1114. Relay R2 couples inductionmotor 1104 to three-phase output power 1120 produced by inverter 1108.

DSP 1106 operates induction motor 1104 directly across three-phase ACline-frequency power 1114 by opening relay R2 and closing relay R1.Relay R2 isolates inverter 1108 and first motor 1102 from three-phase ACline-frequency power 1114. Relay R1 couples induction motor 1104directly to three-phase AC line-frequency power 1114.

In an alternative embodiment, first motor 1102 is an induction motor.Drive circuit 1100 may further include a third relay (not shown) coupledto an output of inverter 1108. In this embodiment, drive circuit 1100enables operation of first motor 1102 and induction motor 1104 in athird mode, where first motor 1102 and induction motor 1104 are bothcoupled directly across three-phase AC line-frequency power 1114.

DSP 1106 operates first (induction) motor 1102 and induction motor 1104directly across three-phase AC line-frequency power 1114 by opening thethird relay and closing relays R1 and R2. Relays R1 and R2 couple firstmotor 1102 and induction motor 1104 directly to three-phase ACline-frequency power 1114. The third relay isolates inverter 1108 fromthree-phase AC line-frequency power 1114 for protection purposes. It isto be understood that in embodiments where first motor 1102 is a PMmotor, it will always be coupled to and driven by inverter 1108.Accordingly, the third relay is not necessary when first motor 1102 is aPM motor. It is also to be understood that the third relay is not anecessary element of the implementation as keeping the inverter powerelectronic switches in the OFF state would achieve similarfunctionality.

FIG. 12 is a schematic diagram of an exemplary drive circuit 1200 for afirst induction motor 1202 in parallel with a second induction motor1204. Drive circuit 1200 includes a DSP 1206 coupled to and configuredto control an inverter 1208 and relays R1 and R2. In some embodiments,the states of relays R1 and R2 may be commanded by a system controllerthat also transmits a command signal to DSP 1106. In other embodiments,control of relays R1 and R2 may be shared between DSP 1106 and thesystem controller.

DSP 1206 determines how to operate first induction motor 1202 and secondinduction motor 1204 based on a detected load 1212 on the motors. DSP1206, in certain embodiments, may quantify load 1212 based onmeasurements external to drive circuit 1200, such as, for example,ambient air temperature in an HVAC system. In alternative embodiments,DSP 1206 may quantify load 1212 based on power output to first inductionmotor 1202 and second induction motor 1204, such as, for example,monitoring current delivered to first induction motor 1202 and secondinduction motor 1204.

Drive circuit 1200 enables operation of first induction motor 1202 andinduction motor 1204 in one of two modes, and further enables transitionamong those modes. First induction motor 1202 and second induction motor1204 may be driven in parallel using variable speed by inverter 1208, ordirectly across a three-phase AC line-frequency power 1214, which may besupplied, for example, by a three-phase generator or a utility.

DSP 1206 controls inverter 1208 to operate first induction motor 1202using either vector control or scalar control. DSP 1206 controls theconfiguration of relays R1 and R2 to select a mode of operation of firstinduction motor 1202 and second induction motor 1204. Relays R1 and R2may be any suitable power switching devices suitable for coupling anddecoupling AC power sources, such as three-phase AC line-frequency power1214 or inverter 1208, to first induction motor 1202 and secondinduction motor 1204.

DSP 1206 operates first induction motor 1202 and second induction motor1204 in parallel by closing relay R2 and opening relay R1. Relay R1,when opened, operates to isolate inverter 1208, first induction motor1202, and second induction motor 1204 from three-phase AC line-frequencypower 1214. Closing relay R2 couples three-phase output power 1220produced by inverter 1208 to both first induction motor 1202 and secondinduction motor 1204.

DSP 1206 operates first induction motor 1202 and second induction motor1204 directly across three-phase AC line-frequency power 1214 by openingrelay R2 and closing relay R1. Opening relay R2 isolates inverter 1208from three-phase AC line-frequency power 1214. Closing relay R1 couplesfirst induction motor 1202 and second induction motor 1204 directly tothree-phase AC line-frequency power 1214.

In certain embodiments, drive circuit 1200 may operate one or moreadditional induction motors in parallel with first induction motor 1202and second first induction motor 1204 and using DSP 1206 and inverter1208.

FIG. 13 is a flow chart illustrating a fluid flow control method 1300for controlling a drive circuit having one or more motors coupled inparallel. FIG. 14 is a schematic diagram of an exemplary drive circuitfor a paralleled first motor, an induction motor, a third motor, and afourth motor. Method 1300 may be used for controlling drive circuit 1100(shown in FIG. 11), drive circuit 1200 (shown in FIG. 12), or drivecircuit 1400. However, method 1300 will be described herein withreference only to drive circuit 1400 of FIG. 14.

Method 1300 is described herein as being applied in controlling afour-motor chiller. Although a four-motor chiller is described, it is tobe understood that the number of motors may be scaled as necessary orexpanded out for use in other applications.

In the exemplary embodiment, method 1300 facilitates controlling two ormore parallel-coupled motors, such as first motor 1402 and inductionmotor 1404, a third motor 1405, and a fourth motor 1407 in variousoperating modes using combinations of using a single inverter 1408 andAC line-frequency power 1414 for higher output. Third and fourth motors1405 and 1407 are parallel-coupled and are selectively coupleable acrossAC line-frequency power 1114 using additional relay R3.

Method 1300 facilitates operating the motors in various operating statesincluding operating one or more motors using the inverter, operatingsome combination of the motors across line power and others using theinverter, and operating all motors across line power. Method 1300enables using a single inverter for driving two or more of the motors,eliminating the necessity of having to purchase drive circuits sized forfull power that are more expensive, drives designed for lower efficiencymotors, and/or drives that require additional customer tuning andspecialized installation, and enables operating one or more additionalmotors using line power.

In the exemplary embodiment, method 1300 includes receiving 1310, by DSP1406, a demand signal indicating a command for the motor outputs. Thisdemand could take the form of a torque demand, a speed demand, or afluid flow demand. The demand signal may be received from a systemcontroller, a thermostat, user input, or the like.

Based on the demand signal, DSP 1406 determines 1320 a state ofoperation of a plurality of predefined states of operation in which tooperate drive circuit 1400 to generate the commanded output. Each stateof operation specifies which of the four motors are activated and ofthose activated, which are operated using inverter 1408 and which areconnected directly to AC line-frequency power 1414.

For example, in a first state of operation associated with a first, orlowest, demand, only first motor 1402 is operated, and is operated usinginverter 1408. This enables operation of first motor 1402 at a desiredspeed that less-than-full speed of first motor 1402 to generate a firstamount of fluid flow.

In a second state of operation associated with a second demand that ishigher than the first demand, first motor 1402 and induction motor 1404are operated, both using inverter 1408. Operating first motor 1402 andinduction motor 1404 in parallel at less-than-full speed facilitatesgenerating a second amount of fluid flow that is higher than the firstamount of fluid flow from the first state of operation.

In a third state of operation associated with a third demand that ishigher than the second demand, first motor 1402 is operated usinginverter 1408 and induction motor 1404 is operated directly across ACline-frequency power 1414. Operating first motor 1402 at less-than-fullspeed and induction motor 1404 at full speed facilitates generating athird amount of fluid flow that is higher than the second amount offluid flow from the second state of operation.

In a fourth state of operation associated with a fourth demand that ishigher than the third demand, first motor 1402 and induction motor 1404are operated using inverter 1408 and third motor 1405 is operateddirectly across AC line-frequency power 1414. Operating first motor 1402and induction motor 1404 at less-than-full speed and third motor 1405 atfull speed facilitates generating a fourth amount of fluid flow that ishigher than the third amount of fluid flow from the third state ofoperation.

In a fifth state of operation associated with a fifth demand that ishigher than the fourth demand, first motor 1402 is operated usinginverter 1408, and induction motor 1404 and third motor 1405 areoperated directly across AC line-frequency power 1414. Operating firstmotor 1402 at less-than-full speed in combination with induction motor1404 and third motor 1405 at full speed facilitates generating a fifthamount of fluid flow that is higher than the fourth amount of fluid flowfrom the fourth state of operation.

In a sixth state of operation associated with a sixth demand that ishigher than the fifth demand, first motor 1402 is operated usinginverter 1408, and induction motor 1404, third motor 1405, and fourthmotor 1407 are operated directly across AC line-frequency power 1414.Operating first motor 1402 at less-than-full speed in combination withinduction motor 1404, third motor 1405, and fourth motor 1407 at fullspeed facilitates generating a sixth amount of fluid flow that is higherthan the fifth amount of fluid flow from the fifth state of operation.

In a seventh state of operation associated with a seventh, or highest,demand that is higher than the sixth demand, first motor 1402, inductionmotor 1404, third motor 1405, and fourth motor 1407 are all operateddirectly across AC line-frequency power 1414. Operating all of firstmotor 1402, induction motor 1404, third motor 1405, and fourth motor1407 at full speed facilitates generating a seventh amount of fluid flowthat is higher than the sixth amount of fluid flow from the sixth stateof operation.

To apply the determined state of operation to the motors, one or more ofthe relays are activated, or closed, to couple one or more of the motorsto either inverter 1408 or AC line-frequency power 1414 as defined bythe particular state of operation. In the exemplary embodiment, therelays are controlled either by DSP 1406 or an external systemcontroller.

Based on the received fluid flow demand signal, DSP 1406 adjusts 1340operation of inverter 1408 to output three-phase voltage to first motor1402 and in some operating states, induction motor 1404, to generate thecommanded amount of fluid flow.

The methods and systems described herein may be implemented usingcomputer programming or engineering techniques including computersoftware, firmware, hardware or any combination or subset thereof,wherein the technical effect may include at least one of: (a) enablingoperation of parallel induction or PM motors with a single inverter ordrive device; (b) improving operating efficiency of parallel motorsthrough use of PM motors; (c) reducing complexity of drive circuits forparallel induction or PM motors; (d) reducing cost of parallel inductionor PM motor applications; and (e) reducing configuration andinstallation complexity of parallel motor applications.

Some embodiments involve the use of one or more electronic or computingdevices. Such devices typically include a processor, processing device,or controller, such as a general purpose central processing unit (CPU),a graphics processing unit (GPU), a microcontroller, a reducedinstruction set computer (RISC) processor, an application specificintegrated circuit (ASIC), a programmable logic circuit (PLC), a fieldprogrammable gate array (FPGA), a digital signal processing (DSP)device, and/or any other circuit or processing device capable ofexecuting the functions described herein. The methods described hereinmay be encoded as executable instructions embodied in a computerreadable medium, including, without limitation, a storage device and/ora memory device. Such instructions, when executed by a processingdevice, cause the processing device to perform at least a portion of themethods described herein. The above examples are exemplary only, andthus are not intended to limit in any way the definition and/or meaningof the terms processor, processing device, and controller.

In the embodiments described herein, memory may include, but is notlimited to, a computer-readable medium, such as a random access memory(RAM), and a computer-readable non-volatile medium, such as flashmemory. Alternatively, a floppy disk, a compact disc-read only memory(CD-ROM), a magneto-optical disk (MOD), and/or a digital versatile disc(DVD) may also be used. Also, in the embodiments described herein,additional input channels may be, but are not limited to, computerperipherals associated with an operator interface such as a mouse and akeyboard. Alternatively, other computer peripherals may also be usedthat may include, for example, but not be limited to, a scanner.Furthermore, in the exemplary embodiment, additional output channels mayinclude, but not be limited to, an operator interface monitor.

As used herein, the terms “software” and “firmware” are interchangeable,and include any computer program stored in memory for execution by aprocessor, including RAM memory, ROM memory, EPROM memory, EEPROMmemory, and non-volatile RAM (NVRAM) memory. The above memory types areexamples only, and are thus not limiting as to the types of memoryusable for storage of a computer program.

The systems and methods described herein are not limited to the specificembodiments described herein, but rather, components of the systemsand/or steps of the methods may be utilized independently and separatelyfrom other components and/or steps described herein.

This written description uses examples to provide details on thedisclosure, including the best mode, and also to enable any personskilled in the art to practice the disclosure, including making andusing any devices or systems and performing any incorporated methods.The patentable scope of the disclosure is defined by the claims, and mayinclude other examples that occur to those skilled in the art. Suchother examples are intended to be within the scope of the claims if theyhave structural elements that do not differ from the literal language ofthe claims, or if they include equivalent structural elements withinsubstantial differences from the literal language of the claims.

What is claimed is:
 1. A method of operating a drive circuit forparallel electric motors, at least one of which is a permanent magnet(PM) motor, said method comprising: receiving measurements of statorphase currents in the parallel electric motors; selecting a target PMmotor, from among the parallel electric motors, that generates a largesttorque output; executing a vector control algorithm to generate acomplex command voltage vector for the target PM motor; generating andtransmitting a pulse width modulation (PWM) signal based on the complexcommand voltage vector for controlling an inverter; and operating theinverter according to the PWM signal to supply three-phase alternatingcurrent (AC) power to the parallel electric motors.
 2. The method ofclaim 1, wherein selecting a target PM motor comprises: comparingrespective stator phase currents for the parallel electric motors; anddetermining which of the parallel electric motors is drawing thegreatest torque producing current through its stator windings.
 3. Themethod of claim 1, wherein operating the inverter further comprisessupplying the three-phase AC power to at least one parallel PM motorother than the target PM motor.
 4. The method of claim 3, whereinsupplying the three-phase AC power to the at least one parallel PM motorfurther comprises dissipating excess current in the form of anadditional flux current.
 5. The method of claim 1, wherein operating theinverter further comprises supplying the three-phase AC power to atleast one parallel induction motor.
 6. The method of claim 1 furthercomprising: receiving at least one stator phase current measurement; andcomputing at least one stator phase current measurement based on the atleast one received stator phase current measurement.
 7. A drive circuitfor parallel electric motors, at least one of which is a permanentmagnet (PM) motor, said drive circuit comprising: an inverter coupled tothe parallel electric motors and configured to supply an alternatingcurrent (AC) signal to stator windings thereof based on a PWM signal;and a digital signal processor (DSP) coupled to said inverter andconfigured to: receive respective stator phase current measurements forthe parallel electric motors; select a target PM motor, of the parallelelectric motors, having a largest torque output among the parallelelectric motors; execute a vector control algorithm to generate acomplex command voltage vector for the target PM motor; generate the PWMsignal based on the complex command voltage vector for the target PMmotor; and transmit the PWM signal to said inverter to operate saidinverter and supply the AC signal to the stator windings of the parallelelectric motors.
 8. The drive circuit of claim 7, wherein said DSP isfurther configured to select the target PM motor based on the respectivestator phase current measurements for the target PM motor exceeding therespective stator phase current measurements for each other PM motoramong the parallel electric motors.
 9. The drive circuit of claim 7,wherein said DSP is further configured to: receive at least one statorphase current measurement for a given electric motor among the parallelelectric motors; and compute at least one other stator phase currentmeasurement for the given electric motor.
 10. The drive circuit of claim7, wherein said inverter comprises three phase legs for generating athree-phase AC output power to be delivered to the parallel electricmotors.
 11. The drive circuit of claim 10, wherein said DSP is furtherconfigured to generate respective PWM signals for controlling each ofsaid three phase legs.
 12. The drive circuit of claim 7, wherein saidDSP, when executing the vector control algorithm, is further configuredto: determine a rotor position for the target PM motor; determine arotor angle for the target PM motor based on the rotor position,respective stator phase currents for the target PM motor, and anelectrical parameter of the stator windings of the target PM motor;transform the respective stator phase currents for the target PM motorinto a flux-torque coordinate system in a rotating rotor reference framebased on the rotor angle; compute commanded flux and torque voltagecomponents based on the transformed respective stator phase currents forthe target PM motor; and compute the complex command voltage vectorbased on the commanded flux and torque current components.
 13. The drivecircuit of claim 7, wherein said inverter is coupled to at least oneinduction motor among the parallel electric motors, and wherein saidinverter is further configured to supply the AC signal to the statorwindings thereof based on the PWM signal.
 14. The drive circuit of claim7, wherein said inverter, in supplying the AC signal to the statorwindings of the parallel electric motors other than the target PM motor,is further configured to dissipate excess current in the stator windingsin the form of an additional flux current.
 15. The drive circuit ofclaim 13, wherein said DSP is further configured to selectively operateat least one induction motor in parallel with the target PM motorthrough said inverter.
 16. The drive circuit of claim 15, wherein saidDSP is further configured to selectively operate the at least oneinduction motor using line frequency power.
 17. A system, comprising: afirst three-phase motor configured to drive a load; an inverter coupledto said first three-phase motor and configured to supply a three-phaseoutput power thereto; an induction motor configured to drive the load,and further configured to be selectively coupled in parallel to saidfirst three-phase motor and said inverter; a three-phase ACline-frequency bus configured to be selectively coupled to saidinduction motor; and a digital signal processor (DSP) coupled to saidinverter and configured to: selectively, when in a direct operatingmode, couple said induction motor to said three-phase AC line-frequencybus, and decouple said induction motor from said first three-phase motorand said inverter; selectively, when in a parallel operating mode,couple said induction motor to said inverter, and decouple saidinduction motor from said three-phase AC line-frequency bus; controlsaid inverter to supply the three-phase output power from said inverterto said first three-phase motor; and control said inverter to supply thethree-phase output power from said inverter to said induction motor whenin the parallel operating mode.
 18. The system of claim 17, wherein saidDSP is further configured to select one of the direct operating mode andthe parallel operating mode based on a quantification of the load. 19.The system of claim 17, wherein said first three-phase motor comprises apermanent magnet motor.
 20. The system of claim 17, wherein said firstthree-phase motor comprises a second induction motor.
 21. The system ofclaim 20, wherein said DSP is further configured to selectively, when inthe direct operating mode, couple said second induction motor to saidthree-phase AC line-frequency bus.
 22. The system of claim 21 furthercomprising a third induction motor configured to drive the load, and afourth induction motor configured to drive the load, the third andfourth induction motors configured to be selectively coupled in parallelto said three-phase AC line-frequency bus.
 23. The system of claim 22,wherein said inverter has an operating capacity that is less than theaggregate operating capacity of all motors configured to drive the load.24. A method of controlling fluid flow generated by fluid-movingequipment driven by a plurality of motors, said method comprising:receiving a fluid flow demand signal indicating an amount of fluid flowcommanded to be output by the fluid-moving equipment driven by theplurality of motors; determining a state of operation, of a plurality ofpredefined states of operation, in which to operate a drive circuitcoupled to the plurality of motors to generate the commanded amount offluid flow; adjusting at least one relay associated with at least onemotor of the plurality of motors to activate and selectively couple theat least one motor to an inverter or an AC line-frequency poweraccording to the determined state of operation; and adjusting operationof the inverter to generate the commanded amount of fluid flow.